1. Field of the Invention
The present invention relates to a radio frequency amplifying apparatus capable of being applied to a power amplifying module of a communications system, or the like, and having a protection voltage varying function.
2. Description of the Related Art
Generally, as a wireless communications scheme, a digital modulation and demodulation scheme appropriate in view of improving frequency use efficiency has been used. For example, a quadrature phase shift keying (QPSK) scheme has been used in a code division multiple access (CDMA) type portable phone, and an orthogonal frequency division multiplexing (OFDM) digital modulation scheme has been used in a wireless local area network (WLAN) according to standard 802.11 set by the institute of electrical and electronics engineers (IEEE) (IEEE communications standard 802.11).
A wireless communications system using such wireless communications schemes includes a power amplifying apparatus used in order to amplify a radio frequency signal.
Here, in a system requiring linear amplification, a power amplifying apparatus having linearity in order to amplify a transmission signal without distortion is required. Here, linearity means that even in the case in which a level of power of an input signal is changed, power of an output signal is amplified at a predetermined ratio and a phase thereof is not changed.
Further, in a wireless communications apparatus, a technology of compensating for a temperature so that a power amplifying ratio is not significantly changed, even in the case that ambient temperature is changed, is required.
In addition, a radio frequency amplifying apparatus is used in order to amplify a weak signal in the communications system. Particularly, the radio frequency amplifying apparatus may be used in a transmitter in order to amplify power of a transmission signal.
The radio frequency amplifying apparatus as described above includes at least one amplifying transistor. In the case in which a voltage higher than a withstand voltage able to be withstood by an amplifying transistor is introduced into the amplifying transistor, the amplifying transistor may break down.
In order to prevent the amplifying transistor from breaking down as described above, a protection circuit capable of performing an overvoltage protection function may be added.
The protection circuit as described above serves to block an overvoltage by performing the protection function in a case in which an overvoltage higher than a predetermined voltage is introduced into the amplifying transistor.
The protection circuit used in an existing radio frequency amplifying apparatus may not adjust a reference voltage for determining an overvoltage once the reference voltage is determined at the time of manufacturing the radio frequency amplifying apparatus.
Therefore, once the radio frequency amplifying apparatus is manufactured, a protection voltage is determined in advance, such that it may not be changed. As a result, it is difficult to use the radio frequency amplifying apparatus in other systems, such that there may be a limitation in the system in which the radio frequency amplifying apparatus may be used.
The following Related Art Document relates to a semiconductor integrated circuit and does not disclose a feature in which a protection voltage may be varied.